The present invention relates to conductive catalytic particles and a process for production thereof, a gas-diffusing catalytic electrode, and an electrochemical device.
The conventional gas-diffusing catalytic electrode is produced from catalytic particles (in the form of conductive carbon powder carrying platinum thereon as a catalyst), a water-repellant resin (such as fluorocarbon resin), and an ionic conductor, which are formed into a sheet as disclosed in Japanese Patent Laid-open No. Hei 5-36418 or applied to a carbon sheet.
The electrode thus produced may be used as an electrode for hydrogen decomposition as a constituent of a fuel cell of solid polymer type or the like. In this case, the catalyst (such as platinum) ionizes fuel, giving rise to electrons, which flow through the conductive carbon. The catalyst also ionizes hydrogen, giving rise to protons (H+), which flow into the ionic conducting membrane through the ionic conductor. These actions need interstices for passage of gas, carbon that conducts electricity, an ionic conductor that conducts ions, and a catalyst (such as platinum) to ionize fuel and oxidant.
A usual way to make carbon powder (i.e., as a conductive powder) support platinum (i.e., as a catalyst) thereon is by dipping carbon powder in a solution containing platinum, such as in the form of ions, which is followed by reduction and thermal treatment. The thus processed carbon powder carries platinum fine particles on the surface thereof as disclosed, for example, in Japanese Patent No. 2879649.
The conventional process mentioned above, however, has the disadvantage of requiring steps for reduction and thermal treatment. With thermal treatment at an inadequately low temperature, it renders platinum poor in crystallinity, which results in failure of obtaining favorable catalytic characteristics.
As mentioned above, the catalyst such as platinum ionizes fuel, giving rise to electrons, which flow through the conductive carbon. The catalyst also ionizes hydrogen, giving rise to protons (H+), which flow to the ion conducting membrane through the ionic conductor. Therefore, it is necessary that the carbon powder and the ionic conductor be kept in contact with each other. To this end, the carbon powder carrying platinum is usually coated with an ionic conductor. Unfortunately, the platinum which has been isolated from gas by the ionic conductor is not functional any longer because it functions only at its specific parts in contact with gas.
An alternative way is to coat the carbon powder with an ionic conductor and then load the coated carbon powder with platinum. This process, however, still needs thermal treatment to improve platinum in crystallinity. Thermal treatment at temperatures high enough for desired crystallinity deteriorates the ionic conductor which is usually poor in heat resistance.
FIG. 24A is a schematic sectional view showing a conductive catalytic particle that is produced by the conventional process, which consists of a carbon particle (conductive powder 1) and platinum particles 27 supported thereon. Also, FIG. 24B is a schematic sectional view showing a conductive catalytic particle, which is produced by coating a carbon particle with an ionic conductor 11 and then loading the coated carbon particle with platinum 27 thereon.
As shown in FIG. 24A, the conductive catalytic particle supporting platinum obtained from a liquid phase has platinum 27 thereon in spherical form. Therefore, the platinum 27 readily separate from the surface of carbon particle. Moreover, production in this manner requires a large amount of platinum. In addition, the platinum 27 in spherical form performs its catalytic function only on its surface but does not function inside. Therefore, it has a low catalytic efficiency for its quantity. Another problem is that the platinum 27 enters pores existing in the surface of the carbon particle (not shown.) Thus, the platinum 27 partly remains unfunctional and hence is poor in catalytic efficiency for its quantity.
In the case where the carbon powder is coated with the ionic conductor 11 and then the coated carbon powder is loaded with the platinum 27, as shown in FIG. 24B, the resulting product needs thermal treatment to improve the platinum 27 in crystallinity, as mentioned above. Unfortunately, the ionic conductor 11 is usually poor in heat resistance and becomes deteriorated when heated at a sufficiently high temperature to improve the platinum 27 in crystallinity.
In order to address the problems, a gas-diffusing catalytic electrode which produces a good catalytic action with a small catalytic amount is disclosed in Japanese Patent Application No. 2000-293517.
This provides that physical vapor deposition, such as sputtering as shown in FIG. 25, makes platinum (as a catalyst) adhere to the surface of the carbon powder 1 (as a conductive powder), thereby giving rise to conductive catalytic particles in which the platinum 27 adheres to the surface of the carbon powder 1, as shown in FIG. 25.
In other words, physical vapor deposition yields conductive catalytic particles in which the platinum 27 adheres only to the surface of the carbon powder, as shown in FIG. 26A. The resulting product produces a good catalytic action with a small amount. In addition, the platinum 27 has a sufficiently large area for contact with gas. In other words, the platinum 27 has a large specific surface area that contributes to reaction and hence has improved catalytic performance.
As shown in FIG. 26B, the catalytic particle is composed of a carbon particle 1, an ionic conductor 11 adhering to the surface thereof, and platinum 27 adhering to the surface thereof. Since the platinum 27 is attached by physical vapor deposition, the resulting product does not need thermal treatment to improve the crystallinity of platinum, unlike the conventional process. Therefore, the platinum 27 can be attached without deteriorating the performance of the ionic conductor 11.
The advantage of coating the conductive powder with a catalyst by physical vapor deposition (such as sputtering) is that the resulting product has a highly pure catalyst (as compared with that obtained by the conventional chemical process) if the target for sputtering is selected from highly pure catalytic materials. However, the highly pure catalyst adhering to the surface of the conductive powder has the disadvantage of being subject to sintering between the catalytic particles. In other words, the catalytic particles used for a fuel cell exhibit high catalytic activity in the early stage. However, they decrease in catalytic activity due to sintering as temperature increases with the lapse of time. Another problem with sintering is that a fuel cell decreases in output if it has a gas-diffusing catalytic electrode based on the conductive catalytic particles containing a highly pure catalyst.
The catalyst increases in catalytic activity as the crystal particles decease in particle diameter. In the case of physical vapor deposition, the initial particle diameter of the catalyst is determined when the catalyst adheres to the conductive powder. However, a highly pure catalyst tends to increase in the crystal particle diameter. In order to obtain a catalyst with a high purity and a high catalytic activity, it is necessary to study how to reduce the crystal particle diameter of the catalyst.
The present invention relates to conductive catalytic particles and a process for production thereof, a gas-diffusing catalytic electrode, and an electrochemical device.